Taxi mag mount antenna for scanning

Re: Taxi mag mount antenna for scanning

as 50cm is approx. 3/4 wavelength at 446 Mhz the original antenna as received would in theory be closer to resonance at 446 than 45cm

Re: Taxi mag mount antenna for scanning

Posted: Thu Jun 28, 2018 6:53 am

by Chris P

as 50cm is approx. 3/4 wavelength at 446 Mhz the original antenna as received would in theory be closer to resonance at 446 than 45cm

Re: Taxi mag mount antenna for scanning

Posted: Thu Jun 28, 2018 9:04 am

by Southwales

As I want it for CB FM,mil air and pmr should I maybe not trim it?.

Re: Taxi mag mount antenna for scanning

Posted: Thu Jun 28, 2018 9:24 am

by Chris P

No point in trimming it as no antenna will be effective over that bandwidth. simple quarterwave whips cut for each band required ( but not practical for CB) and fitted into the base when required will be far more productive .

if someone could have designed an effective wideband mobile antenna to cover the bands you want they would have made a fortune , unfortunately its just not possible although some marketeering managers claim they have .

PS a 1/4 wave whip at 446 is approx. 168mm on CB it is approx. 2750 mm . Even the milair band is too wide to be adequately covered by one whip although one 250 mm long is a good compromise

Re: Taxi mag mount antenna for scanning

Posted: Thu Jun 28, 2018 8:03 pm

by G4RMT

The facts are quite simple - CB reception (and 10m and below ham bands) are pretty rubbish on any form of short non-resonant antenna. They are, to be certain - merely bit off metal in the sky, and they work poorly. As we've said - as soon as you move away from resonance the performance drops off. However, the drop off is not critical when it comes to aircraft - a rubber duck, which is a very poor antenna in it's helical wound format does fine - I can hear surprisingly long distances because the source is line of sight. My Marine band VHF van antenna receives the UHF ham repeater (and gets into it) from Lowestoft to GT Yarmouth - around 8-9 miles or so.

As for the dual band whip - it's similar to one I had and gives a small bit of gain at UHF and VHF ham bands, but less gain elsewhere. Dual band antennas generally perform worse as sc manner antennas because they are optimised for gain, and you don't get something for nothing, and the VSWR rises sharply. Mine was useless at marine band, and UHF business band - the simple whip worked better - because it's just a bit of metal with a much more gentle response curve away from optimum.

There is no way I'd put a dual band ham whip on my van because the results would limit what I can do. My work needs access to multiple bands that I hold licences for and the Marine band optimised whip gets used all the time - the other antennas on the van have had not been connected for a while now.

Re: Taxi mag mount antenna for scanning

Posted: Fri Jun 29, 2018 4:16 pm

by Southwales

Thank you for everyone's help, mine arrived earlier today so will give it a try out later today. Out of interest I have one of these Moonraker mini CB antennas, https://www.moonraker.eu/antennas/cb-ra ... cb-antennaWith it only being 67cm tall can it possibly be any good at CB FM and if so should I maybe have used this as my compromise antenna?My only thought being though that the last time I tried it plugged into my CB, my scanner was picking up better using my Nagoya mini mag, the Moonraker CB antenna was centred on the roof and the mini mag on one corner of the roof, On my CB I could only hear one person but the scanner with mini mag was also picking up the people he was talking to, this confused me as surely a CB antenna should have outperformed the mini mag on CB FM.

Re: Taxi mag mount antenna for scanning

Posted: Fri Jun 29, 2018 6:05 pm

by m0lsx

Southwales wrote: This confused me as surely a CB antenna should have outperformed the mini mag on CB FM.

Yes, but this was a Moonraker antenna & my own experience with Moonraker has been that only one antenna out of an order of three arrived working. And I have heard many others with their own stories about this company & their build quality. Plus many about their customer service. Of the three antennas I ordered was a three band white stick & had VSWR off the scale. This was because a capacitor had not been soldered properly. A Handheld dual band antenna was fortunately checked on a SWR meter before being used, because the SMA connector had been fitted so badly it did not make a proper connection with the radio. Thus showing off the scale VSWR & I cannot remember what the third antenna was at the moment, but that worked. Moonraker replaced both antennas, but I do remember making an effort to replace all 3 Moonraker antennas ASAP after that.

Re: Taxi mag mount antenna for scanning

Posted: Sat Jun 30, 2018 11:22 pm

by Southwales

Tried out the taxi antenna Friday night and Saturday morning, great at picking up taxis but did not receive much else, not sure if this was because it is the weekend and the airwaves are quiet anyway, still not sure if I am best trimming to 45cm as suggested or leave as is.

Re: Taxi mag mount antenna for scanning

Posted: Sun Jul 01, 2018 4:47 pm

by G4RMT

Leave it and keep listening. An inch or two really makes so little difference on receive. If you think about it, aviation antennas are tuned for mid band, but could spend all their lives at an airfield close to one end - it really isn't that critical.